Many private schools are becoming increasingly exclusive and failing to live
up to their original charitable purpose, according to Lord Adonis.

Lord Adonis, the former schools minister, suggested it is inexcusable for the independent sector to separate itself from state education, and called on fee-paying schools to become involved in the academies programme.

In comments ahead of the London Festival of Education, Lord Adonis said that the trustees and governors of private schools should look at their charitable values as a ''matter of conscience and duty''.

''To those in the private school world who are reluctant to embrace academies, I appeal to their professionalism and their charitable missions,'' Lord Adonis said.

''It was excusable to stand apart from state-funded education when the State did not want them engaged in the first place. But that is the isolationist politics of the past.

''With the academies programme, supported across the political spectrum, they have an opportunity to engage in state-funded education without compromising their independence, renewing for the 21st century their essential moral and charitable purposes.''

Lord Adonis argued that leading private schools including Winchester, Charterhouse, Westminster School, Harrow and Eton were all set up for charitable reasons - mainly the education of poor scholars.

''I could go on through the founding charters of hundreds of private schools,'' he said.

''It shouldn't take the Charity Commission to challenge private school foundations about their charitable missions.

''Their trustees and governors should look to them constantly as a matter of conscience and duty. With each passing decade many of these schools have become more not less exclusive, and for generations now, few of them have done anything radical to reconnect with their charitable purposes.

''Most of them are seeking to provide a few more bursaries. But this is hardly enough when they could also be running academies whose central purpose is the very mission for which their assets were originally intended.''

Lord Adonis served as schools minister under the last labour government and is seen as one of the main architects of the academies programme.

Academies are semi-independent state schools with freedom over areas such as the curriculum and staff pay and conditions.

Education Secretary Michael Gove has opened up the programme to allow all schools in England to apply for academy status.

Matthew Burgess, general secretary of the Independent Schools Council (ISC), said: ''Lord Adonis draws attention to our sector's longevity and, in so doing, demonstrates why many are right to be wary of Government-sponsored, totemic policies - particularly when they are painted as moral duties.

''The courts recognised last year, in upholding our judicial review of the Charity Commission, that there is no 'one size fits all' model of charitable engagement.

''The diversity of the sector simply means that there can be no single moral compass pointing unwaveringly in the direction of the Government's academy programme. Instead, there is a rich variety of ways in which schools live up to their responsibility to reach out and serve those who do not pay fees.

''In 2011/12, ISC schools supported almost 40,000 children on means-tested bursaries with an annual value of £284 million; and over 1,000 ISC schools were working in partnership with state schools and their local communities.''

Dr Christopher Ray, chairman of the Headmasters' and Headmistresses' Conference (HMC) and high master of the Manchester Grammar School said: ''There is no 'Berlin Wall' as Lord Adonis puts it. Independent schools are involved in constructive partnerships with state-maintained schools previously acknowledged by the Prime Minister as being immensely important.

''Many independent schools are sceptical about the current preferred approach: the sponsorship of academies - remembering that during 15 years governments of the day have changed direction several times concerning what they believe the independent sector should do to support their specific political aims for education.

''HMC understands the frustration that this and previous governments must feel that their own approaches to correcting the problems inherent in the state system have not been entirely successful and are sympathetic to calls for our continuing assistance.

''However, it is entirely a matter for each school to determine precisely how it should maximise its own benefit to the public, through local partnerships, the provision of means-tested bursaries, the establishment of free schools, the sponsorship of or strong partnerships with academies, the sharing of expertise and facilities, or the many other proven ways of developing further the education of children in this country.''

In 2008, Winchester joined a partnership to support a new academy in Midhurst, West Sussex.

Tony Little, headmaster of Eton College, said: ''Eton College has been doing its duty for many, many years by providing substantial financial support to those who cannot afford the fees and by developing new initiatives with state schools all the time.''